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<a name="boost.proto._"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2><span class="refentrytitle">Struct _</span></h2>
<p>boost::proto::_ — A wildcard grammar element that matches any expression, and a transform that returns
          the current expression unchanged.</p>
</div>
<h2 class="refsynopsisdiv-title">Synopsis</h2>
<div class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../../proto/reference.html#header.boost.proto.matches_hpp" title="Header &lt;boost/proto/matches.hpp&gt;">boost/proto/matches.hpp</a>&gt;

</span>
<span class="keyword">struct</span> <a class="link" href="_.html" title="Struct _">_</a> <span class="special">:</span> <span class="keyword"></span> <a class="link" href="transform.html" title="Struct template transform">proto::transform</a>&lt;_&gt; <span class="special">{</span>
  <span class="comment">// types</span>
  <span class="keyword">typedef</span> <span class="identifier">_</span> <a name="boost.proto._.proto_grammar"></a><span class="identifier">proto_grammar</span><span class="special">;</span>

  <span class="comment">// member classes/structs/unions</span>
  <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <a class="link" href="../../Expr.html" title="Concept Expr">Expr</a><span class="special">,</span> <span class="keyword">typename</span> State<span class="special">,</span> <span class="keyword">typename</span> Data<span class="special">&gt;</span> 
  <span class="keyword">struct</span> <a class="link" href="_/impl.html" title="Struct template impl">impl</a> <span class="special">:</span> <span class="keyword"></span> <a class="link" href="transform_impl.html" title="Struct template transform_impl">proto::transform_impl</a>&lt;Expr, State, Data&gt; <span class="special">{</span>
    <span class="comment">// types</span>
    <span class="keyword">typedef</span> <span class="identifier">Expr</span> <a class="link" href="_/impl.html#boost.proto._.impl.result_type"><span class="identifier">result_type</span></a><span class="special">;</span>

    <span class="comment">// <a class="link" href="_/impl.html#id-1_3_32_5_32_2_1_1_4_4-bb">public member functions</a></span>
    <span class="identifier">Expr</span> <a class="link" href="_/impl.html#id-1_3_32_5_32_2_1_1_4_4_1-bb"><span class="keyword">operator</span><span class="special">(</span><span class="special">)</span></a><span class="special">(</span><span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">expr_param</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">state_param</span><span class="special">,</span> 
                    <span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">data_param</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
  <span class="special">}</span><span class="special">;</span>
<span class="special">}</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="id-1.3.32.5.34.4.4"></a><h2>Description</h2>
<p>
            The wildcard type, <code class="computeroutput">proto::_</code>, is a grammar element such
            that <code class="computeroutput"><a class="link" href="matches.html" title="Struct template matches">proto::matches</a>&lt;E, proto::_&gt;::value</code>
            is <code class="computeroutput">true</code> for any expression type <code class="computeroutput">E</code>.
          </p>
<p>
            The wildcard can also be used as a stand-in for a template argument when matching terminals.
            For instance, the following is a grammar that will match any
            <code class="computeroutput">std::complex&lt;&gt;</code> terminal:</p>
<pre class="programlisting"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">(</span><span class="special">(</span>
  <a class="link" href="matches.html" title="Struct template matches">proto::matches</a><span class="special">&lt;</span>
    <a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="special">&gt;</span><span class="special">::</span><span class="identifier">type</span><span class="special">,</span>
    <span class="bold"><strong><a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span> <span class="special">&gt;</span></strong></span>
  <span class="special">&gt;</span>
<span class="special">)</span><span class="special">)</span><span class="special">;</span></pre>
<p>
          </p>
<p>
            When used as a transform, <code class="computeroutput">proto::_</code> returns the current expression
            unchanged. For instance, in the following, <code class="computeroutput">proto::_</code> is used with
            the <code class="computeroutput"><a class="link" href="fold.html" title="Struct template fold">proto::fold&lt;&gt;</a></code>
            transform to fold the children of a node:</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CountChildren</span> <span class="special">:</span>
  <a class="link" href="or_.html" title="Struct template or_">proto::or_</a><span class="special">&lt;</span>
    <span class="comment">// Terminals have no children</span>
    <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</span><a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">&gt;</span><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span><span class="special">(</span><span class="special">)</span><span class="special">&gt;</span><span class="special">,</span>
    <span class="comment">// Use proto::fold&lt;&gt; to count the children of non-terminals</span>
    <a class="link" href="otherwise.html" title="Struct template otherwise">proto::otherwise</a><span class="special">&lt;</span>
      <a class="link" href="fold.html" title="Struct template fold">proto::fold</a><span class="special">&lt;</span>
        <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <span class="comment">// &lt;-- fold the current expression</span>
        <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span><span class="special">(</span><span class="special">)</span><span class="special">,</span>
        <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><a class="link" href="_state.html" title="Struct _state">proto::_state</a><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span><span class="special">(</span><span class="special">)</span>
      <span class="special">&gt;</span>
    <span class="special">&gt;</span>
  <span class="special">&gt;</span>
<span class="special">{</span><span class="special">}</span><span class="special">;</span></pre>
<p>
          </p>
</div>
</div>
<div class="copyright-footer">Copyright © 2008 Eric Niebler<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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